Method for producing water-soluble or water-swellable polymerizates with a very low residual monomer content, products produced according to this method and the use thereof

ABSTRACT

The invention relates to a process for producing water-soluble or water-swellable polymer products from acrylic acid and/or acrylic acid derivatives partially or completely neutralized with nitrogen compounds and with exceedingly low levels of residual monomers, which process is characterized by subsequent heating of the polymer product at temperatures of from 120 to 240° C. The polymers produced accordingly are suitable for use as flocculants, dispersants and absorbers.

REFERENCE TO PRIOR APPLICATIONS

This application is a national stage application, under 35 U.S.C. 371,of PCT Application PCT/EP98/07290, filed Nov. 13, 1998, which claimspriority to German Application No. 197 52 128.2, filed Nov. 25, 1997.

The invention relates to a process for producing synthetic polymersbased on acrylic acid and derivatives thereof, which polymers have anexceedingly low content of residual monomers. The polymers have highmolecular weights and either are water-soluble or have high absorptivecapacity for water, aqueous solutions and body fluids.

Various synthetic polymer products having absorptive capacity for waterand body fluids have been described in numerous patents: e.g.crosslinked polymers and copolymers based on acrylic or methacrylic acid(U.S. Pat. No. 4,018,951, U.S. Pat. No. 4,066,583, U.S. Pat. No.4,062,817, U.S. Pat. No. 4,066,583, DE-OS 26 13 135, DE 27 12 043, DE 2813 634), or acrylamidopropanesulfonic acid copolymers (DE 31 24 008).These absorbents are virtually insoluble in water, absorbing a multipleof their weight of water, urine or other aqueous solutions atequilibrium. In addition to high absorptive capacity, other propertiesof synthetic absorbents such as low residual monomer content, low ratioof water solubles and high gel strength of the swollen polymer particlesare also mentioned in some patent specifications.

In the production of high molecular weight water-soluble orwater-swellable, i.e., partially crosslinked polymers and mixedpolymers, complete conversion of the monomers—in particular, monomersbased on acrylic acid—has been found impossible. on an industrial scale,residual monomer levels of from 0.1 to 0.5 wt.-% normally are observedin the polymer product.

The toxicity of the monomers remaining in the polymer product iswell-known and therefore, a process for producing polymers and mixedpolymers virtually free of monomers would be highly convenient. Up tonow, since it has not been possible as yet to conduct a polymerizationprocess in such a way that virtually no residual monomers would remain,it has been attempted to remove the toxic residual monomers from thepolymer product by converting them into harmless derivatives. Thus,DE-AS 1,070,377 and U.S. Pat. No. 2,960,486 use mixing the aqueoussolutions of high molecular weight acrylamide polymers with sodiumdisulfite solution and drying at 80-120°. One precondition for thesemethods is processing the polymer product in a highly diluted (2-3%)polymer solution, which is uneconomic, and for this reason these methodshave rarely been applied in practice.

The treatment of a polymer gel with an aqueous solution of sodiumbisulfite or metabisulfite has been described in U.S. Pat. No.3,755,280, and with a solid alkali sulfite in EP 175,554, where residualmonomer contents of from 0.03 to 0.3 wt.-% have been obtained. Likewise,the JP-PS 56/103207 uses bisulfites, sulfites and pyrosulfites for thesame purpose. U.S. Pat. No. 3,780,006 uses gaseous sulfur dioxide inorder to decrease the acrylamide concentration in an emulsion polymer.EP 505,163 uses treatment of polymer products with a combination ofmetabisulfite and a surface-active agent (HLB from 3 to 40) subsequentto completed polymerization, thereby decreasing the residual monomercontent down to 10 ppm. From 2 to 5 wt.-% of metabisulfite (relative tothe polymer gel having 40% w.s., that is, from 5 to 12.5 wt.-% ofmetabisulfite relative to the dry polymer product) is necessary in sucha secondary treatment of polymer gels in order to achieve the desireddecrease in residual monomers. Such high quantities of added substancesmay have highly negative effects on the application-technicalproperties.

EP 303,518 A2 describes a process for producing absorbent polymerproducts based on acrylic acid/acrylamide, and according to thisinvention, a high degree of neutralization of from 70 to 100 mole-%, ahigh monomer concentration of at least 50%, and a combination ofthermally decomposing azo and redox initiators are used. Owing to theseconditions, the reaction proceeds in such a way that all of the waterused in the batch undergoes vaporization even during the polymerization,so that subsequent drying can be omitted, and the residual monomercontents are said to drop below 500 ppm, preferably below 200 ppm. Theresidual monomer levels of the experimental examples have not beenspecified.

DE 37 24 709 Al describes a process for producing polymer productshaving low residual monomer content, wherein the polymer productsfollowing production are reacted in a swollen form as a gel or insolution with compounds reacting with the double bonds in the residualmonomers at 50-150° C.

In U.S. Pat. No. 4,766,173, the decrease of acrylic acid residualmonomer in polymer products is effected by subsequent treatment of thepolymer products using amino acids of the lysine and cysteine types attemperatures from 80° C. on.

WO 94/20547 describes additives such as bromate and chlorate in thepolymerization solution and subsequent heating of the final polymerwherein, inter alia, a decrease of the residual monomers is effected bythe additives. The bromates and chlorates may also be added subsequentto the polymerization. Despite these measures, the residual monomercontent of the polymer products is between about 135 and 1100 ppm.

While the prior art processes described above permit a substantialdecrease of residual monomers, they involve several drawbacks such asmalodors as a result of liberated sulfur dioxide, corrosion of the plantby sulfur oxide resultant products (e.g. sulfurous acid and sulfuricacid as well as salts thereof in acidic medium). Above all, however, anysubsequent treatment of the final polymer that has been producedrepresents an additional technological step necessitating additionalapparatus and a quite considerable input of time, and also, a quitesignificant amount of additive remains in the final product and mayadversely affect the application-technical properties.

It is therefore the object of the invention to produce synthetic polymerproducts of per se known chemical composition with high molecular weightor high retention capability for water, aqueous liquids and body fluids,using particular specific conditions so as to obtain final productshaving an extremely low residual monomer content and goodapplication-technical properties, without necessitating a subsequenttreatment of the produced polymers using additional chemical compounds.

Surprisingly, it has now been found that synthetic polymer productsbased on acrylic acid and having the desired properties and an extremelylow content of residual monomers can be produced by free-radicalpolymerization in aqueous solution, provided at least one basic nitrogencompound such as ammonia is used in the partial or completeneutralization of acid monomer components during the preparation of themonomer mixture to be polymerized, and the polymer products aresubjected to subsequent heating at 120 to 240° C., preferably 140 to180° C. The polymer products according to the process of the inventionhave an extremely low content of residual monomers, preferably beingbelow 50 ppm, particularly below 30 ppm. Of special importance is thefact that acrylamide, which is rated critical in toxicological terms,can be decreased to a residual content of below 10 ppm.

For example, ammonia or ammonium hydroxide, aliphatic mono- andpolyamines, particularly aliphatic C₁-C₁₀ amines, cycloaliphatic mono-and polyamines, particularly cycloaliphatic C₆-C₁₂ amines, aromaticmono- and polyamines, particularly aromatic C₆-C₁₂ amines, heterocyclicamines, hydroxylamine and alkanolamines such as monoethanolamine anddiethanolamine or mixtures thereof are used as basic nitrogen compoundsfor neutralization. Particularly preferred nitrogen compounds areammonia or ammonium hydroxide, ethanolamine and diethanolamine.

The nitrogen compounds for complete or partial neutralization of theacid monomer components can be employed in the form of their free bases.In this context, part of the acid monomers may also be neutralized byother bases, and the nitrogen compound may be present in a molar deficitrelative to these other bases. Advantageously, the nitrogen compound forneutralizing the acid monomer component can be used for neutralizationlevels ranging from 10 to 100%.

In order to achieve an economic production of the polymers, thepolymerization is carried out using initiators which decompose atrelatively low temperatures. Inorganic or organic peroxides or redoxsystems are used as common initiators. When polymerizing in aqueousphase, inorganic peroxides such as peroxodisulfates are frequently usedalone or in combination with a reducing component.

The polymerization can be initiated using a redox initiator system or bymeans of photopolymerization. The redox initiator system mostly consistsof two components, i.e., an inorganic or organic peroxide-containingcompound and a reducing component such as sulfite, hydrosulfite,thiosulfate, sulfinic acid, ascorbic acid and their copper, ferrous ormanganese salts. Alkali metal or ammonium peroxides such as potassiumperoxodisulfate and hydrogen peroxide can be used as inorganic peroxidecompounds, and benzoyl peroxide, butyl hydroperoxide as organicperoxides. In general, the polymerization is initiated using a redoxinitiator system. Other initiators may be used in addition to such aninitiator system. In the case of photopolymerization which can beinitiated by UV-containing light, so-called photoinitiators like e.g.benzoin or benzoin derivatives such as benzoin ethers, benzil andderivatives thereof such as benzil ketals, acrylic diazonium salts, azoinitiators such as 2,2′-azobisisobutyronitrile,2,2′-azo-bis(2-amidinopropane) hydrochloride or acetophenone derivativesare used. The amounts of peroxide-containing component and reducingcomponent may range from 0.0005 to 0.5 wt.-%, preferably from 0.001 to0.1 wt.-% (relative to the monomer solution), and the amounts ofphotoinitiators may range from 0.001 to 0.1 wt.-%, preferably from 0.002to 0.05 wt.-% (relative to the monomer solution).

Predominantly, possible monomers are acrylic acid, acrylamide,methacrylic acid, and methacrylamide, which arc polymerized to formhomopolymers or mixed polymers, but also other monomers such asacrylonitrile, methacrylonitrile, N,N-dimethylacrylamide,vinylpyrrolidone, vinylpyridine, vinyl acetate, as well as otherwater-soluble polymerizable acids and salts thereof, particularlymaleic, fumaric, itaconic, vinylsulfonic, oracrylamidomethylpropanesulfonic acid; also, hydroxy group-containingesters of polymerizable acids, particularly the hydroxyethyl andhydroxypropyl esters of acrylic and methacrylic acid can be used;furthermore, amino group-containing and ammonium group-containing estersand amides of polymerizable acids, such as dialkylamino esters,particularly the dimethyl and diethylaminoalkyl esters of acrylic andmethacrylic acid, as well as trimethyl and trimethylammoniumalkyl estersand the corresponding amides. The above monomers can be polymerizedalone to form homopolymers or mixed with each other to form mixedpolymers. In addition, small amounts of water-insoluble monomers can becopolymerized with the above monomers, e.g. esters of acrylic and/ormethacrylic acid with C₁-C₁₀ alcohols, styrene and alkylated styrenes.In general, the proportion of water-soluble monomers is from 60 to 100wt.-%, relative to the overall monomers. As a rule, the water-insoluble(hydrophobic) monomers make up for 0 to 40 wt.-% of the monomers.

Small amounts of crosslinking monomers, e.g. monomers having more thanone reactive group in their molecule can be polymerized together withthe above-mentioned monomers, thereby forming partially crosslinkedpolymer products which are no longer soluble in water but merelyswellable. Bi- or multifunctional monomers, e.g. amides such asmethylenebisacrylamide or -methacrylamide or ethylene-bisacrylamide, andalso, esters of polyols and alkoxylated polyols, such as diacrylates ortriacrylates, e.g. butanediol or ethylene glycol diacrylate, polyglycoldi-(meth)acrylates, trimethylolpropane triacrylate, di- and triacrylateesters of trimethylolpropane preferably oxyalkylated (ethoxylated) with1 to 30 mol alkylene oxide, acrylate and methacrylate esters of glyceroland pentaerythritol, or of glycerol and pentaerythritol preferablyoxyethylated with 1 to 30 mol ethylene oxide, and also, allyl compoundssuch as allyl(meth)acrylate, alkoxylated allyl(meth)acrylate preferablyreacted with 1 to 30 mol ethylene oxide, triallyl cyanurate, diallylmaleate, polyallyl esters, tetraallyloxyethane, triallylamine,tetraallylethylenediamine, allyl esters of phosphoric acid orphosphorous acid, and crosslinkable monomers such as N-methylolcompounds of amides such as methacrylamide or acrylamide and ethersderived therefrom may be mentioned as crosslinking monomers. The ratioof crosslinking monomers is from 0 to 10 wt.-%, preferably from 0.01 to3.0 wt.-%, relative to the overall monomers.

The water-absorbing polymeric absorbents are obtained using at least onecrosslinker. As crosslinkers, those compounds are used which contain atleast 2 or more functional groups (double bonds, epoxy groups) and arecapable of being incorporated into the growing polymer chains duringpolymerization. As a result, crosslinking sites are formed at variouspositions in the polymer product, which hold together the individualpolymer chains, taking such an effect that the polymer particles canmerely swell in a liquid and will not dissolve in said liquid. Theproperties of the crosslinked polymer are determined by the chemicalstructure of the crosslinker, the number of crosslinking sites, butalso, by their distribution in the polymer chains. In case of optimumincorporation of crosslinker into the polymer, crosslinked polymerproducts will be formed having their crosslinking sites uniformlydistributed, so that non-crosslinked regions or even non-crosslinked(i.e., water-soluble) low molecular weight components are barely presentin the polymer product. Uniform distribution of the crosslinking siteswithin the polymer results in a product which also has optimum retentioncapability for aqueous liquids and optimum gel strength in the swollenstate.

The polymerization preferably is performed in aqueous solution,batchwise in a polymerization vessel or continuously on a continuousbelt, e.g. according to DE 35 44 770. Given a virtually adiabatic courseof polymerization, an aqueous polymer gel is formed at a correspondinginitial concentration of from 15 to 50 wt.-% of monomers. By selectingthe initial monomer concentration and an appropriate low initialtemperature ranging from 0 to 50° C., preferably from 5 to 25° C., it ispossible to conduct the polymerization in such a way that the maximumtemperature in the aqueous polymer gel being formed can easily becontrolled.

According to the process of the invention, the produced polymer isheated at a temperature of 120-240° C., preferably 140-180° C. in orderto achieve said low residual monomer content. Care must be taken not todeteriorate other essential properties of the polymer product by anexcessively high temperature or long time. The optimum time required forheating depends on the nitrogen compound concentration and temperaturelevel and can be determined using just a few experiments. In most of thecases, a time period between 10 minutes and 2 hours is sufficient,preferably between 10 minutes and 1 hour.

The polymer products are classified in screen fractions required forvarious applications. For example, when using the crosslinked polymerproducts as absorbents in the hygiene sector, grain size distributionsof from 0.2 to 1 mm are preferred, products ranging from 0.2 to 3 mm arepreferably used in the agricultural sector, and products ranging from0.1 to 1.2 mm for soluble polymers in the flocculant sector.

As a result of the low content of residual monomers, the polymerproducts according to the invention have the following advantages overwell-known polymers produced according to prior art:

-   1. The polymer products can be used in manufacturing absorbent    materials where an extremely low content of residual monomers is    demanded for toxicological reasons, e.g. in superabsorbers for the    hygiene industry (diapers, incontinence products), food industry    (packagings), in soil improvers, etc..-   2. The polymer products can also be used in the controlled release    of other substances subsequently incorporated in the polymer or    contained in the polymer to other bodies, e.g. in dosing drugs in    human medicine, in dosing nutrients (fertilizer effect) to plants,    insecticides and herbicides in aqueous media, preferably on large    water surfaces, with no major quantities of toxicologically critical    residual monomers invading the environment.-   3. Non-crosslinked, water-soluble polymer products having low    residual monomer content may also be used in such applications where    low residual monomer levels are demanded for toxicological reasons,    e.g. in drinking water treatment, when used as thickening agents in    the food industry, in dosing of drugs, etc..

The incorporation of nutrients for plants, of herbicides, insecticides,disinfectants, drugs, antibacterial agents and other substances into thepolymer product can be effected by directly adding these compounds tothe monomer solution as long as the course of polymerization is notperturbed thereby. However, in case these substances do have an impacton the polymerization, their incorporation has to be effected not beforeduring polymerization or after completed polymerization by includingthese substances in the ready-produced polymer gel according to e.g. DE40 29 591, DE 40 29 592 or DE 40 29 593.

EXAMPLES

Absorptive Capacity Determination

1 g of polymer is added with stirring to a 250 ml beaker containing 200ml of Fertilizer solution*. Following a stirring period of 15 minutes,the magnetic stirrer is turned off, and this is allowed to stand for 45minutes. The content of the beaker then is poured over a screen having amesh width of 0.3 mm, where the polymer swollen by the fertilizersolution remains on the screen. The amount of fertilizer solutionabsorbed by the polymer is calculated in ml/g of polymer from thedifference between the 200 ml amount of solution originally employed andthe amount of solution having passed the screen.

* Peters® Professional Fertilizer, Grace-Sierra Horticultural ProductsComp., Pennsylvania, USA.

Determination of the Residual Monomer Content

The residual monomer components acrylamide and acrylic acid aredetermined in aqueous extracts of the polymers using HPLC with internalstandards.

Example 1

0.9 g of methylenebisacrylamide was initially dissolved in 275 g ofwater in a polymerization vessel and mixed with 301 g of acrylic acid.Thereafter, the monomer solution was neutralized with 168 g of sodiumhydroxide solution (50%) and 118 g of ammonia (25%), cooled to 10° C.and purged with nitrogen. Following addition of the initiator solutions(1.0 g of sodium peroxodisulfate, 0.2 g of2,2′-azobis(2-methylpropionamidine)dihydrochloride, each in 20 ml ofwater, and 0.05 g of Irgacure® 651 (photoinitiator, Ciba Geigy Company)in 2 g of acrylic acid), the polymerization was initiated using UVlight. Thorough mixing of the initiator solutions with the monomersolution is one precondition for homogeneous polymerization throughoutthe polymer block. The maximum temperature of 103° C. (in awell-insulated polymerization vessel) was reached within 15 minutes.After completed polymerization, the polymer was heated at 160° C. for1.5 hours and milled to the desired grain fraction. A crosslinkedacrylic acid homopolymer as sodium/ammonium salt had been formed. Thescreened grain fraction from 200 to 1000 μm was used for furtherinvestigations. The absorptive capacity for a 0.1% fertilizer solutionwas 103 ml/g, and the residual monomer content was 20 ppm of acrylicacid.

Example 2

Under similar conditions as in Example 1, 0.6 g ofmethylenebisacrylamide was dissolved in 275 g of water and mixed with301 g of acrylic acid. Thereafter, the monomer solution was neutralizedwith 261 g of potassium hydroxide solution (50%) and 118 g of ammonia(25%), cooled to 10° C. and purged with nitrogen. Following addition ofthe initiator solutions as in Example 1, the polymerization wasinitiated using UV light. The maximum temperature of 102° C. was reachedwithin 10 minutes. After completed polymerization, the polymer washeated at 180° C. for 1.5 hours and milled to the desired grainfraction. A crosslinked acrylic acid homopolymer as potassium/ammoniumsalt had been formed. The screened grain fraction from 200 to 1000 μmwas used for further investigations. The absorptive capacity for a 0.1%fertilizer solution was 110 ml/g, and the residual monomer content was12 ppm of acrylic acid.

Example 3

0.6 g of methylenebisacrylamide was initially dissolved in 290 g ofwater in a polymerization vessel and mixed with 208 g of acrylic acidand 223 g of acrylamide solution (40%). Thereafter, the monomer solutionwas neutralized with 180 g of potassium hydroxide solution (50%) and39.3 g of ammonia (25%), cooled to 10° C. and purged with nitrogen.Following addition of the initiator solutions as in Example 1, thepolymerization was initiated using UV light. The maximum temperature of104° C. was reached within 8 minutes. After completed polymerization,the polymer was heated at 160° C. for 1.5 hours and milled to thedesired grain fraction. A crosslinked acrylic acid/acrylamide copolymeras potassium/ammonium salt had been formed. The screened grain fractionfrom 200 to 1000 μm was used for further investigations. The absorptivecapacity for a 0.1% fertilizer solution was 120 ml/g, and the residualmonomer content was 10 ppm of acrylic acid and 10 ppm of acrylamide.

Example 4

Under similar conditions as in Example 3, a terpolymer of acrylamide,acrylic acid and acrylamido(2-methylpropane)sulfonic acid (AMPS) at aratio of 50/49.3/0.7 mole-% was produced. 50% of the acrylic acid wasneutralized with potassium hydroxide solution and 40% with ammonia(25%). After completed polymerization, the polymer was heated for 1.5hours at 140° C. and 160° C., respectively. The absorptive capacity fora 0.1% fertilizer solution was 110 and 108 ml/g, respectively, and theresidual monomer content was 28 ppm of acrylic acid and 9 ppm ofacrylamide or 15 ppm of acrylic acid and 7 ppm of acrylamide.

Example 5

0.6 g of methylenebisacrylamide was dissolved in 230 g of water in apolymerization vessel and mixed with 118.6 g of acrylic acid and 446.4 gof acrylamide (40%). Thereafter, the monomer solution was neutralizedwith 102.6 g of potassium hydroxide solution (50%) and 46.1 g of ammonia(25%), cooled to 10° C. and purged with nitrogen. Following addition ofthe initiator solutions according to Example 1, the polymerization wasinitiated using UV light. The maximum temperature of 102° C. was reachedwithin 8 minutes. After completed polymerization, the polymer was heatedat 160° C. for 1.5 hours and milled to the desired grain fraction. Acrosslinked acrylic acid/acrylamide copolymer as potassium/ammonium salthad been formed. The absorptive capacity for a 0.1% fertilizer solutionwas 106 ml/g, and the residual monomer content was 15 ppm of acrylicacid and 10 ppm of acrylamide.

Examples 6 to 35

146.5 g of acrylic acid, 372 g of acrylamide solution (40%) and 6.9 g ofAMPS were initially mixed in 230 g of water in a polymerization vessel.The monomer solution was added with varying amounts of variouscrosslinkers, so that polymers having varying crosslinking levels wereformed. Methylenebisacrylamide (MBA), triallylamine (TAA) andtrimethylolpropane (TMPTA) alone or in combination were used ascrosslinkers. Thereafter, the monomer solution was neutralized eitherwith 128.5 g of potassium hydroxide solution (45%) and 56 g of ammonia(25%) (Examples 6 to 14) or with 126 g of ammonia and with 84.2 g ofammonia (25%), respectively (Examples 15 to 35), so as to result in anoverall neutralization level of 90% and 60%, respectively. The monomersolution was cooled to 10° C. and purged with nitrogen. Followingaddition of the initiator solutions according to example 1, thepolymerization was initiated using UV light. The maximum temperature of102-104° C. was reached within 8-12 minutes. After completedpolymerization, the polymer was heated at 140° C. for 1.5 hours andmilled to the desired grain fraction. Crosslinked acrylicacid/acrylamide/AMPS terpolymers as potassium/ammonium or ammonium saltshad been formed.

Grain fractions of from 200 to 1000 μm were used for furtherinvestigations. Amounts of crosslinker, absorptive capacities andresidual monomer levels can be inferred from the following Table.

Residual Amount crosslinker Neutralization Absorptive capacity monomerMBA TMPTA TAA KOH NH₃ 0.1% Fertilizer soln. AcA AcS Ex. (% rel. w.s.)(%) (ml/g) (ppm)  6 0.1 50 40 142  4 15  7 0.25 50 40 120  5 11  8 0.350 40 112  5 20  9 0.3 50 40 113  4 25 10 0.5 50 40  98  6 40 11 0.4 5040 157  5 30 12 0.8 50 40 146  7 25 13 0.1 0.2 50 40 135  5 30 14 0.10.4 50 40 112  6 25 15 0.2  0 60 128  8 10 16 0.25  0 60 120 11 11 170.3  0 60 114 16 11 18 0.4  0 60 108 13 10 19 0.2 0.1  0 60 118 20 15 200.2 0.2  0 60 120 37 30 21 0.2 0.4  0 60 111 19 10 22 0.1 0.4  0 60 120 6  5 23 0.1 0.8  0 60 112  5  5 24 0.2 0.2  0 60 118 16 20 25 0.2 0.4 0 60 115 20 15 26 0.2  0 90 130  7 33 27 0.25  0 90 120  6 15 28 0.3  090 120  8 13 29 0.2 0.1  0 90 134  7 12 30 0.2 0.2  0 90 130  8 38 310.2 0.4  0 90 126  9 11 32 0.1 0.8  0 90 115  9 13 33 0.2 0.2  0 90 110 7 35 34 0.2 0.4  0 90 106  9 12 35 0.2 0.8  0 90 104 11 20

Example 36

The monomer solution according to Example 6 having 0.9 g ofmethylenebisacrylamide was neutralized with 128 g of potassium hydroxidesolution (45%) to a neutralization level of 50%. Following addition of25.1 g of ethanolamine, the monomer solution was cooled to 10° C. andpurged with nitrogen. Following addition of the initiator solutionsaccording to Example 1, the polymerization was initiated using UV light.The maximum temperature of 104° C. was reached within 8 minutes. Aftercompleted polymerization, the polymer was heated at 160° C. for 1.5hours and milled to the desired grain fraction. The absorptive capacityfor a 0.1% fertilizer solution was 48 ml/g, and the residual monomercontent was 20 ppm of acrylic acid and 17 ppm of acryl-amide.

Example 37

The monomer solution according to Example 6 having 0.9 g ofmethylenebisacrylamide was neutralized with 120 g of potassium hydroxidesolution (45%) to a neutralization level of 50%. Following addition of50.2 g of ethanolamine, the monomer solution was cooled to 10° C. andpurged with nitrogen. Following addition of the initiator solutionsaccording to Example 1, the polymerization was initiated using UV light.The maximum temperature of 104° C. was reached within 6 minutes. Aftercompleted polymerization, the polymer was heated at 160° C. for 1.5hours and milled to the desired grain fraction. The absorptive capacityfor a 0.1% fertilizer solution was 46 ml/g, and the residual monomercontent was 20 ppm of acrylic acid and 11 ppm of acrylamide.

Example 38

148 g of acrylic acid, 372 g of acrylamide solution (40%), 0.6 g ofmethylenebisacrylamide, and 1.2 g of triallylamine were initiallydissolved in 260 g of water in a polymerization vessel. Thereafter, themonomer solution was neutralized with 126 g of ammonia (25%) to aneutralization level of 90%. The monomer solution then was cooled to 10°C. and purged with nitrogen. Following addition of the initiatorsolutions (0.8 g of ABAH, 1.0 g of sodium peroxodisulfate, 0.2 g ofhydrogen peroxide, and 0.05 g of ascorbic acid, each in 5 ml of water),the polymerization was initiated. The maximum temperature of 100° C. wasreached within 10 minutes. After completed polymerization, the polymerwas heated at 140° C. for 1.5 hours and milled to the desired grainfraction. A crosslinked acrylic acid/acrylamide copolymer as ammoniumsalt had been formed. The absorptive capacity for a 0.1% fertilizersolution was 132 ml/g, and the residual monomer content was 10 ppm ofacrylamide and 25 ppm of acrylic acid.

Example 39

Initially, 146 g of acrylic acid, 372 g of acrylamide solution (40%) and14 g of AMPS (50%) in 300 g of water were mixed in a polymerizationvessel. Thereafter, the monomer solution was neutralized with 126 g ofammonia (25%) to a neutralization level of 90%. The monomer solutionthen was cooled to 10° C. and purged with nitrogen. Following additionof the initiator solutions according to Example 1, the polymerizationwas initiated using UV light. The maximum temperature of 100° C. wasreached within 10 minutes. After completed polymerization, the polymerwas heated at 140° C. for 1.5 hours and milled to the desired grainfraction. A high molecular weight, water-soluble acrylic acid/acrylamidecopolymer with a residual monomer content of 3 ppm of acrylic acid and 5ppm of acrylamide had been formed. A 0.1% aqueous solution of thispolymer had a Brookfield viscosity of 400 mPa.s.

Example 40

Initially, 146 g of acrylic acid was mixed with 493 g of water in apolymerization vessel. Thereafter, the monomer solution was neutralizedwith 168 g of ammonia (25%) to a neutralization level of 60%. Themonomer solution then was cooled to 10° C. and purged with nitrogen.Following addition of the initiator solutions according to Example 1,the polymerization was initiated using UV light. The maximum temperatureof 102° C. was reached within 10 minutes. After completedpolymerization, the polymer was heated at 140° C. for 1.5 hours andmilled to the desired grain fraction. A high molecular weight,water-soluble acrylic acid polymer with a Brookfield viscosity of 240mPa.s and a residual monomer content of 11 ppm of acrylic had beenformed.

Example 41

Example 40 was repeated; however, the polymer was heated to 160° C.instead of 140° C. The polymer product had a Brookfield viscosity of 350mPa.s, and the residual monomer content was 18 ppm of acrylic acid.

1. A process for producing a water-soluble or water-swellable polymer orcopolymer comprising: providing an acid monomer alone, or in combinationwith at least one comonomer; partially or completely neutralizing saidat least one monomer with at least one basic nitrogen compound;free-radical polymerizing said at least one monomer alone, or incombination with said at least one comonomer to form said water-solubleor water-swellable polymer or copolymer, wherein said free-radicalpolymerization is started at a temperature of from 0 to 50° C and isperformed in aqueous solution at a maximum temperature of no more than102-104° C, to provide an aqueous polymer or copolymer solution or anaqueous polymer or copolymer gel; and, subsequent to completion of saidfree-radical polymerization, heating said water-soluble orwater-swellable polymer or copolymer at a temperature of from 120° to240° ; wherein said heating is caried out for a time period between 10minutes and 2 hours; wherein the water-soluble or water-swellablepolymer or copolymer has a content of residual monomer of less than 50ppm.
 2. The process according to claim 1, wherein said nitrogen compoundis selected from the group consisting of ammonia, ammonium hydroxide,hydroxylamine, alkanolamines, alkylamines and mixtures thereof.
 3. Theprocess according to claim 1, wherein said nitrogen compound is selectedfrom the group consisting of ammonia, ammonium hydroxide,monoethanolamine, diethanolamine and mixtures thereof.
 4. The processaccording to claim 1, wherein said at least one acid monomer isneutralized with said nitrogen compound up to a neutralization level offrom 10 to 100%.
 5. The process according to claim 1, wherein monomersbased on acrylic acid, methacrylic acid or derivatives of thesecarboxylic acids are subjected to polymerization.
 6. The process ofclaim 1, wherein said comonomer is vinyl acetate.
 7. The processaccording to claim 1, wherein at least one crosslinker based on a bi- orpolyfunctional monomer is used in addition.
 8. The process according toclaim 1, wherein the polymer or copolymer is heated at a temperature offrom 140 to 180° C.
 9. The process according to claim 1, wherein thewater-soluble or water-swellable polymer or copolymer has a content ofresidual monomer of less than 30 ppm.
 10. The process according to claim1, wherein the water-soluble or water-swellable polymer or copolymer hasa content of residual content of acrylamide of less than 10 ppm.
 11. Theprocess according to claim 1, wherein said nitrogen compound is selectedfrom the group consisting of ammonia, ammonium hydroxide, aliphaticmonoamines, aliphatic polyamines, cycloaliphatic monoamines,cycloaliphatic polyamines, aromatic monoamines, aromatic polyamines,heterocyclic amines, hydroxylamine, alkanolamines and mixtures thereof.12. The process according to claim 1, wherein said at least one monomeris selected from the group consisting of acrylic acid, methacrylic acid,maleic acid, fumaric acid, itaconic acid, vinyl sulfonic acid,acrylamidopropanesulfonic acid, and mixtures thereof.
 13. The processaccording to claim 1, wherein said at least one comonomer is selectedfrom the group consisting of acrylonitrile, methacrylonitrile,N,N-dimethylacryl amide, vinylpyrrolidone, vinylpyridine, vinyl acetate,hydroxyl group-containing esters of polymerizable acids, aminogroup-containing esters of polymerizable acids, amino group-containingamides of polymerizable acids, ammonium group-containing esters ofpolymerizable acids, ammonium group-containing amides of polymerizableacids, C₁-C₁₀ alcohol esters of acrylic acid, C₁-C₁₀ alcohol esters ofmethacrylic acid, C₁-C₁₀ alcohol esters of a mixture of acrylic acid andmethacrylic acid, esters of acrylic acid and methacrylic acid withstyrene, esters of methacrylic acid with styrene, esters of methacrylicacid with alkylated styrene, esters of a mixture of acrylic acid andmethacrylic acid with alkylated styrene.
 14. The process according toclaim 8, wherein said heating is carried out for a time period between10 minutes and 1 hour.
 15. The process according to claim 1, whereinsaid heating is carried out for a time period between 10 minutes and 1hour.
 16. The process according to claim 1, wherein said free radicalpolymerization is started at a temperature of from 5 to 25° C.
 17. Theprocess according to claim 1, wherein the polymer or copolymer is heatedat a temperature of 160° C or 180 ° C.
 18. The process according toclaim 1, wherein said heating is carried out for a period between 10minutes and 90 minutes.